[RFC] ARM DMA mapping TODO, v1

[benh@kernel.crashing.org (Benjamin Herrenschmidt)]

On Thu, 2011-04-28 at 10:30 +0100, Russell King - ARM Linux wrote:
> On Thu, Apr 28, 2011 at 07:41:07AM +1000, Benjamin Herrenschmidt wrote:
> > 
> > > As I said above, I don't think bus code can do it.  Take my example
> > > above of a tulip pci device on x86 and a tulip pci device on ARM.  Both
> > > use the same PCI code.
> > > 
> > > Maybe something in asm/pci.h - but that invites having lots of bus
> > > specific header files in asm/.
> > > 
> > > A better solution imho would be to have an architecture callback for
> > > struct device which gets registered, which can inspect the type of
> > > the device, and set the flag depending on where it appears in the
> > > tree.
> > 
> > Now -that's gross :-)
> > 
> > For PCI you can have the flag propagate from the PHB down, for busses
> > without a bus type (platform) then whoever instanciate them (the
> > platform code) can set that appropriately.
> 
> How can you do that when it changes mid-bus heirarchy?  I'm thinking
> of the situation where the DRM stuff is on a child bus below the
> root bus, and the root bus has DMA coherent devices on it but the DRM
> stuff doesn't.

But that's not PCI right ? IE. with PCI, coherency is a property of the
PHB... 

> Your solution doesn't allow that - and I believe that's what Arnd is
> talking about.

Well, for the rest I'm thinking just bolt it into the platform until you
can put the property in the DT :-)

Cheers,
Ben.